Sentience

"They, as well as all additional sentient life within three radii of the galactic center, died as planned."

— 343 Guilty Spark

A sentient being is a being with the ability to think intelligently, commonly determined by the being's ability to speak, manipulate tools, and reason. Other lifeforms lacking such capabilities are classified as non-sentient, though some classification systems have a middle ground, semi-sentient, for species that had very primitive capabilities to manipulate tools, use language, and/or solve problems.

In Relation to the Flood

The Flood only consumed an organism that was sentient and also had a sufficient biomass to support the infection process. For the most part, all species in the Covenant are sentient, but some lack the structural integrity needed to be infected; for example, Hunters cannot be infected because each sentry are made from a buildup of many eels that lack a spinal cord to control.